College Physics

Figure 15.13(a) The upper curve is an isothermal process (ΔT= 0), whereas the lower curve is an adiabatic process (Q= 0). Both start from the same point A,

but the isothermal process does more work than the adiabatic because heat transfer into the gas takes place to keep its temperature constant. This keeps the pressure

higher all along the isothermal path than along the adiabatic path, producing more work. The adiabatic path thus ends up with a lower pressure and temperature at point

C, even though the final volume is the same as for the isothermal process. (b) The cycle ABCA produces a net work output.

Reversible Processes

Both isothermal and adiabatic processes such as shown inFigure 15.13are reversible in principle. Areversible processis one in which both the

system and its environment can return to exactly the states they were in by following the reverse path. The reverse isothermal and adiabatic paths

are BA and CA, respectively. Real macroscopic processes are never exactly reversible. In the previous examples, our system is a gas (like that in

Figure 15.9), and its environment is the piston, cylinder, and the rest of the universe. If there are any energy-dissipating mechanisms, such as friction

or turbulence, then heat transfer to the environment occurs for either direction of the piston. So, for example, if the path BA is followed and there is

friction, then the gas will be returned to its original state but the environment will not—it will have been heated in both directions. Reversibility requires

the direction of heat transfer to reverse for the reverse path. Since dissipative mechanisms cannot be completely eliminated, real processes cannot

be reversible.

There must be reasons that real macroscopic processes cannot be reversible. We can imagine them going in reverse. For example, heat transfer

occurs spontaneously from hot to cold and never spontaneously the reverse. Yet it would not violate the first law of thermodynamics for this to

happen. In fact, all spontaneous processes, such as bubbles bursting, never go in reverse. There is a second thermodynamic law that forbids them

from going in reverse. When we study this law, we will learn something about nature and also find that such a law limits the efficiency of heat engines.

We will find that heat engines with the greatest possible theoretical efficiency would have to use reversible processes, and even they cannot convert

all heat transfer into doing work.Table 15.2summarizes the simpler thermodynamic processes and their definitions.

Table 15.2Summary of Simple

Thermodynamic Processes

Isobaric Constant pressureW=PΔV

Isochoric Constant volumeW= 0

IsothermalConstant temperatureQ=W

Adiabatic No heat transferQ= 0

PhET Explorations: States of Matter

Watch different types of molecules form a solid, liquid, or gas. Add or remove heat and watch the phase change. Change the temperature or

volume of a container and see a pressure-temperature diagram respond in real time. Relate the interaction potential to the forces between

molecules.

518 CHAPTER 15 | THERMODYNAMICS

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